In the field of medicine, imaging and image guidance are a significant component of clinical care. From diagnosis and monitoring of disease, to planning of the surgical approach, to guidance during procedures and follow-up after the procedure is complete, imaging and image guidance provides effective and multifaceted treatment approaches, for a variety of procedures, including surgery and radiation therapy. Targeted stem cell delivery, adaptive chemotherapy regimens, and radiation therapy are only a few examples of procedures utilizing imaging guidance in the medical field.
Advanced imaging modalities such as Magnetic Resonance Imaging (MRI) have led to improved rates and accuracy of detection, diagnosis and staging in several fields of medicine including neurology, where imaging of diseases such as brain cancer, stroke, Intra-Cerebral Hemorrhage (ICH), and neurodegenerative diseases, such as Parkinson's and Alzheimer's, are performed. As an imaging modality, MRI enables three-dimensional visualization of tissue with high contrast in soft tissue without the use of ionizing radiation. This modality is often used in conjunction with other modalities such as Ultrasound (US), Positron Emission Tomography (PET) and Computed X-ray Tomography (CT), by examining the same tissue using the different physical principals available with each modality. CT is often used to visualize boney structures and blood vessels when used in conjunction with an intra-venous agent such as an iodinated contrast agent. MRI may also be performed using a similar contrast agent, such as an intra-venous gadolinium based contrast agent which has pharmaco-kinetic properties that enable visualization of tumors and break-down of the blood brain barrier. These multi-modality solutions can provide varying degrees of contrast between different tissue types, tissue function, and disease states. Imaging modalities can be used in isolation, or in combination to better differentiate and diagnose disease.
In neurosurgery, for example, brain tumors are typically excised through an open craniotomy approach guided by imaging. The data collected in these solutions typically consists of CT scans with an associated contrast agent, such as iodinated contrast agent, as well as MRI scans with an associated contrast agent, such as gadolinium contrast agent. Optical imaging is often used in the form of a microscope to differentiate the boundaries of the tumor from healthy tissue, known as the peripheral zone. Tracking of instruments relative to the patient and the associated imaging data is also often achieved by way of external hardware systems such as mechanical arms, or radiofrequency or optical tracking devices. As a set, these devices are commonly referred to as surgical navigation systems.
During a medical procedure, navigation systems require a registration process to transform between the physical position of the patient in the operating room and the volumetric image set (e.g., MRI/CT) being used as a reference to assist in accessing the target area in the patient. Conventionally, this registration is done relative to the position of a patient reference, which is visible by the tracking system and stays fixed in position and orientation relative to the patient throughout the procedure.
This registration is typically accomplished through a touch-point registration method which involves constructing a correspondence of identifiable points (e.g., either fiducial or anatomic points) between the patient in the operating room and the volumetric image set of the patient. Such an approach to registration has a number of disadvantages, such as those that increase effort on the parts of the surgical team including requiring fiducials to be placed before patient scans, requiring points to be identified one at a time, requiring points to be reacquired. Additionally, disadvantages of this method also affect the accuracy of the guidance system, such as providing for a limited number of points, touch point collection is subject to user variability, and the physical stylus used for collecting the points can deform or deflect patient skin position, in addition the patient is required to be imaged directly before the procedure and the fiducials may move/fall off.
Another approach to performing a registration is the surface trace registration method which involves acquiring a contour of the patient, by drawing a line over the surface of the patient, usually acquiring a series of points, using either a tracked stylus pointer or a laser pointer and fitting that contour to the corresponding extracted surface from an image of the patient.
The patient reference (e.g. a tracked reference marker) is placed in a fixed position relative to the patient anatomy of interest. Generally, this can be accomplished by securing the patient reference to a patient immobilization frame, such as a neurosurgical head holder (skull clamp) system like the MAYFIELD® skull clamp, which is itself rigidly attached to the patient. In the course of a surgical procedure, the patient reference and/or parts of the patient's body may be inadvertently bumped or moved, resulting in a loss of registration. Inaccuracies in image guidance can have serious adverse consequences on the outcome of the clinical procedure. As such, after the detection of a loss of registration, the patient anatomy may need to be re-registered or, in some cases, navigation may be disabled for the remainder of the surgical procedure.